1. Field of the Invention
The invention is directed to a network filter that comprises at least one filter inductance and at least one filter capacitor.
2. Description of the Related Art
Network filters of the species initially cited are known that comprise two through four phase inputs and a protective conductor or a filter housing in which each phase input is connected to the corresponding phase output via a filter inductance. Capacitors can be connected to each side of the filter inductances. These capacitors can be circuited to the protective conductor or to the filter housing either directly or via a star point that is connected to a further capacitor. The known network filters can be employed, for example, to screen a frequency converter connected to the network, i.e., to prevent the feed of high-frequency interference from the frequency converter into the power network. The network filter is connected between the power network and the frequency converter for this purpose
In order to meet standards defined in conjunction with the frequency converter, for example the class A limit value according to EN 55011, the filter must exhibit certain minimum attenuation properties. With respect to this standard, a limit value of 79 dB μV is defined for line-bound interference in the range between 0.15 and 0.5 MHz and a limit value of 73 dB μV (quasi peak value) is defined between 0.5 and 30 MHz. In order to meet these specifications, it is necessary that the filter inductances employed in the known network filters exhibit a relatively high inductance.
In order to realize such filter inductances, “current-compensated” inductors with a ferrite core are usually employed, these being wound such that the magnetic flux induced in the ferrite core by the main current (push-pull) yields a sum of approximately zero. The inductor windings can be composed of one or more turns. Correspondingly many turns must be wound in order to achieve high inductance values.
This manufacturing method has the disadvantage that it is time-consuming and costly and that, in particular, it places narrow limits in view of the mechanical implementation (weight and size) of network filters.
The employment of filter inductances that comprise lower values and that could therefore be produced by simply slipping a ferrite core over a conductor is not possible given the previously mentioned network filters. When the filter inductances are too small, the attenuation properties of the network filter become poorer so that the network filter no longer meets the corresponding standards in conjunction with, for example, a frequency converter connected to it.